You've forgotten about the nasty reality of AGC and receiver overload. For what you say to be true, one must disable the AGC and the receiver must have the dynamic range/overload capability to not fold with the wider bandwidth. If they did we would never need narrow filters. Many rigs have no "off AGC" position. The only other choice becomes reducing the RF gain. That eliminates the weak one you're trying to hear. A narrower filter can mitigate the AGC problem as well as improving the S/N ratio.
The sound card digital filtering comes after all these stages in the chain. It simply cannot make up for receiver generated junk. The dynamic range is not set by the sound card but by the weakest part of the chain-- the RX. A SDR radio where the sound card is the IF is a different story entirely-- if the front end "sound card" and down converter circuitry is linear and can handle strong sigs. There are indeed sound cards that claim a 120db dynamic range. 73 de Brian/K3KO --- In digitalradio@yahoogroups.com, "Vojtech Bubnik" <[EMAIL PROTECTED]> wrote: > > --- In digitalradio@yahoogroups.com, "Brian A" <alsopb@> wrote: > > 1) Using a 200 Hz filter instead of 400 or 500 Hz filter gives a 3db > > S/N ratio improvment-- PSK or RTTY. It's guaranteed. > > It is not. Using narrower filter will reduce total noise and out of > channel QRM, lowering dynamic range requirements for MF, AF and A/D > stages. If the chain has enough dynamic range, it does not matter, > which filter you use. > > Each software PSK31 decoder contains narrow DSP filter just after A/D > What really matters is S/N after this digital filter, which is > independent of MF filter bandwidth. > > > In other words, all the extra baggage (bandwidth) is generally just > > extra weight with no robust benefit. > > There are physical laws telling that one needs less energy to > transport the same information, if he increases channel bandwidth. > > 73, Vojtech OK1IAK >
